1. Field of the Invention
The present invention relates to an ammonia concentration detection sensor.
2. Description of the Related Art
In recent emission control, it has been examined to introduce a certain reducing agent into exhaust gas before reduction treatment from the outside as required as a method for promoting the reduction of NOx because ingredients (CO, HC, and the like) for reducing NOx are insufficient in the exhaust gas of diesel engines. As such a reducing agent, urea has been examined. As the reduction of NOx by urea, NOx is reduced by a catalyst utilizing ammonia generated by hydrolyzing urea, and decomposed into harmless N2 and H2O. Here, in order to control the amount of urea to be hydrolyzed, it is necessary to monitor the concentration of ammonia that has been left in an excessive amount after reducing NOx, and thus a sensor therefor is required. As such a sensor, a gas concentration detection sensor can be utilized that detects the concentration of combustible gas ingredients using an oxygen ion conductor. Such a gas concentration detection sensor has been proposed to have a protective cover that is configured to make the flow of exhaust gas uniform or a protective cover that is configured to prevent the adhesion of condensed water generated when starting the engine around a sensor element unit (e.g., Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-301579